A luminescent material absorbs energy in one portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. A luminescent material in powder form is commonly called a phosphor, while a luminescent material in the form of a transparent solid body is commonly called a scintillator.
Most useful phosphors and scintillators emit radiation in the visible portion of the spectrum in response to the absorption of radiation which is outside the visible portion of the spectrum. Thus, the phosphor performs the function of converting electromagnetic radiation to which the human eye is not sensitive into electromagnetic radiation to which the human eye is sensitive. Most phosphors are responsive to more energetic portions of the electromagnetic spectrum than the visible portion of the spectrum. Thus, there are phosphors and scintillators which are responsive to ultraviolet light (as in fluorescent lamps), electrons (as in cathode ray tubes) and x-rays (as in radiography).
Two broad classes of luminescent materials are recognized. These are self-activated luminescent materials and impurity-activated luminescent materials.
A self-activated luminescent material is one in which the pure crystalline host material upon absorption of a high energy photon elevates electrons to an excited state from which they return to a lower energy state by emitting a photon. Self-activated luminescent materials normally have a broad spectrum emission pattern because of the relatively wide range of energies which the electron may have in either the excited or the lower energy states. Thus, any given excited electron may emit a fairly wide range of energy during its transition from its excited to its lower energy state, depending on the particular energies it has before and after its emissive transition.
An impurity activated luminescent material is normally one in which a non-luminescent host material has been modified by inclusion of an activator species which is present in the host material in a relatively low concentration, such as in the range from about 200 parts per million to 1,000 parts per million. However, some materials require several mole or atomic percent of activator ions for optimized light output. With an impurity activated luminescent material, the activator ions may directly absorb the incident photons or the lattice may absorb the incident photons and transfer the absorbed photon energy to the activator ions.
The photon absorbed by the lattice may create mobile migrating electrons and holes in the lattice. Due to favorable charge configurations, the migrating electrons and holes are trapped at the activator ions, where they recombine and emit a photon of luminescent light.
Alternatively, if the photon is absorbed directly by the activator ion, the photon raises one or more electrons of the activator ion to a more excited state. These electrons, in returning to their less excited state, emit a photon of luminescent light.
In many commonly employed impurity activated luminescent materials, the electrons which emit the luminescent light are d or f shell electrons whose energy levels may be significantly affected or relatively unaffected, respectively, by the surrounding crystal field. In those situations where the activator ion is not much affect by the local crystal field, the emitted luminescent light is substantially characteristic of the activator ions rather than the host material and the luminescent spectrum comprises one or more relatively narrow emission peaks. This contrasts with a self-activated luminescent material's much broader emission spectrum.
When a host lattice absorbs the incident photon (i.e. the excitation energy) and transfers it to the activator ion, the host lattice acts as a sensitizer. The host lattice may also be doped with sensitizer atoms. The sensitizer atoms absorb the incident photon either directly, or from the host lattice, and transfer it to the activator ion.
One prior art green light emitting phosphor is Zn.sub.2 SiO.sub.4 :Mn.sup.2+. This phosphor is used in display devices, such as plasma displays and cathode ray tubes (CRT), and in various fluorescent lamps. The phosphor absorbs the emitted UV radiation from the lamp or plasma display or electrons in a CRT and emits radiation in the green spectral range.
It is generally advantageous for a phosphor to be resistant to radiation damage and exhibit a high lumen maintenance. Radiation damage is the characteristic of a luminescent material in which the quantity of light emitted by the luminescent material in response to a given intensity of stimulating radiation decreases after the material has been exposed to a high radiation does. Lumen maintenance is the ability of a luminescent material to resist radiation damage over time. Luminescent materials with a high resistance to radiation damage over time have a high lumen maintenance.
However, the Zn.sub.2 SiO.sub.4 :Mn.sup.2+ phosphor has shown a significant decrease in light output after several hundred hours of bombardment by energetic UV radiation or electrons. Therefore, the phosphor suffers from poor lumen maintenance.
Two of the current inventors recently proposed a new Sr.sub.1-x Pr.sub.x Al.sub.12-y Mg.sub.y O.sub.19 phosphor in U.S. Pat. No. 5,571,451. This phosphor emits light in the blue spectral range due to emission from the Pr.sup.3+ activator. Furthermore, this phosphor exhibits a high quantum efficiency in the blue spectral range due to a Pr quantum splitting effect. However, this phosphor does not exhibit luminescence in the green spectral range.
In view of the foregoing, it would be desirable to provide a green emitting phosphor or scintillator material that exhibits an adequate lumen maintenance. It would also be desirable to provide a method of making such a phosphor or scintillator.